Molecular sieve catalysts used in a fluidized-bed reactor or a riser reactor will typically have an average particle diameter from 40 μm to 300 μm. Catalyst particle size within this range is needed for proper fluidization as well as to efficiently separate the catalyst from the gaseous products in a cyclone separator. To maintain the desired catalyst diameter the molecular sieve is formulated with other materials. Dilution of the molecular sieve with these materials is also used to control the rate of reaction, control the temperature of the reactor and regenerator, and to stabilize and protect the molecular sieve.
Formulated molecular sieve catalysts present a problem not found in other types of industrial catalysts, that is, how to maintain the physical integrity of the molecular sieve catalyst during the fluidized cyclic process of reaction, separation, and regeneration. The cycles of reaction, separation, and regeneration are carried out at high temperatures and high flow rates. Collisions and abrasions between catalyst particles, between the catalyst particles and reactor walls and between the catalyst particles and other parts of the unit tend to cause physical breakdown of the original catalyst into smaller catalyst particles known as fines. This physical breakdown is referred to as catalyst attrition. The fines usually have particle diameters smaller than 20 microns—much smaller than the original catalyst particles. Catalysts with higher attrition resistance are desirable because, among other reasons, fewer fines are generated for disposal, less environmental impact is caused by unrecoverable airborne particulates, optimal fluidized conditions are maintained, operating costs are lower, and less replacement catalyst is required.
Molecular sieve catalysts are formed by various methods, for example, by spray drying or extruding a slurry containing the molecular sieve and the other catalyst components. The catalysts are formed by mixing the zeolitic molecular sieve with one or more binding agents such as one or more types of alumina and/or silica. Matrix materials, typically clays, are also added and serve as diluents to control the rate of the catalytic reaction, and to facilitate heat transfer during many stages of the process. In U.S. Pat. No. 5,346,875 to Wachter et al. zeolite-Y (21.8 wt %) is mixed with Kaolin clay (14.5 wt %), silica sol (48.3 wt %), and Reheis chlorhydrol (15.4 wt %) to form a slurry which is then spray dried and calcined. A conventional calcination procedure was used; heating at 550° C. in air for 2 hours.
Non-zeolitic, molecular sieve catalysts are known to convert oxygenates, particularly methanol, to light olefins. The oxygenate to olefin process includes separate processing zones for conducting the catalytic reaction, product-catalyst separation, and catalyst regeneration. The produced olefin and other hydrocarbon products are separated from the catalyst particles in a separator, suitably a cyclone separator. A portion of the catalyst is recovered from the separator and passed to a regenerator. In the regenerator the non-zeolitic molecular sieve catalyst contacts a combusting gas, e.g. air, at a temperature sufficient to burn off carbon deposits, commonly referred to a coke, that accumulate on the surface and in the pores of the catalyst. The regenerated catalyst is then returned to the oxygenate conversion reactor.
In this process, the non-zeolitic molecular sieve catalyst is subjected to great mechanical stresses. As the catalyst is transferred from the reaction zone to cyclone separators, to regenerators, and finally back to the reaction zone the catalyst will tend to disintegrate into catalyst fines. These catalyst fines must be removed from the reactor process and discarded. No matter how resistant the catalyst is to attrition, eventually the oxygenate to olefin process will break down the non-zeolitic molecular sieve catalyst because the catalyst moves through the system at such high speeds. The resistance of the catalyst to attrition is an important property of the catalyst.
In PCT Publication No. WO 99/21651 to Wachter et al. and U.S. Pat. No. 4,973,792 to Lewis et al., silicoaluminophosphate (SAPO) molecular sieve catalysts were produced by preparing a slurry containing SAPO-34, Kaolin clay, and Reheis chlorhydrol. The slurry was then directed to a spray dryer to form catalyst particles with the desired size. The spray dried catalysts were calcined, however the conditions of the calcination were stated to be not critical.
In U.S. Pat. Nos. 5,248,647 and 5,095,163 to Barger et al. SAPO molecular sieve is mixed with an aqueous silica sol and spray dried. The spray dried catalyst is mixed with an aqueous solution of ammonium sulfate at 60° C. three times, then washed with water and dried at 100° C. The dried, ion-exchanged catalyst is then calcined in air at 550° C. for over 3.3 hours and then the temperature is lowered to ambient room temperature over a period of 2 hours. A portion of this catalyst is then contacted with steam at 725° C. or 750° C. for 10 hours. Steam treatment following calcination is shown to increase catalyst life, increase selectivity to ethylene and propylene, and decrease selectivity to propane.
If SAPO molecular sieve catalysts are ever going to be used commercially to convert oxygenates to olefins, catalysts with greater attrition properties are needed. For this reason, the Applicants' sought to develop SAPO catalysts with a relatively high resistance to attrition.